Russia says its only active duty soldiers in Ukraine were the 10 captured earlier this week, who Moscow insists had mistakenly wandered across the border.

NATO estimates that as many as 1,000 Russian troops are fighting inside Ukraine with an additional 20,000 soldiers massing on the border for support or possible reinforcements.

"It's important to look at this exchange through the current context," said Rick Roth, the top spokesman for Canadian Foreign Affairs Minister John Baird. "The original tweet was in the context of Russian military personnel being found in sovereign Ukrainian territory, claiming they were lost. Nobody believes that."

Dutch Brig.-Gen. Nico Tak said there has been a significant escalation in both the level and sophistication of Russia's military interference in Ukraine.

"We have also detected large quantities of advanced weapons, including air defense systems, artillery, tanks, and armored personnel carriers being transferred to separatist forces in eastern Ukraine," said Tak.

Baird called the recent events a "significant provocation" in advance of next week's NATO summit in Wales, where the alliance's leaders are expected to hold a special meeting with new Ukrainian President Petro Poroshenko.

Ukraine is not a member of NATO, so the alliance is not automatically obliged to come to its defense.

President Barack Obama ruled out the possibility that the U.S. will take military action over Ukraine and declined to characterize the assault as an invasion.